Papers by Author: Robert E. Dinnebier

Abstract: Molecular motion by refinement of TLS matrices from high resolution laboratory powder diffraction data: implementation in the program TOPAS and application to crystalline naphthalene

Abstract: A set of macros for the powerful least squares and global optimization program TOPAS has been written in order to create a user friendly interface to the maximum entropy method (MEM) program BayMem. As a case study, the crystal structures of pure strontium hydroxyl apatite and strontium hydroxyl apatite with approximately 10% of the hydroxyl anions substituted by peroxide anions were analyzed by TOPAS and BayMem. Although the concentration of peroxide is relatively small, distinct differences in the two crystal structures could be made visible by the MEM.

Abstract: A new program to perform fast sequential and parametric whole powder profile refinement of in situ time-resolved powder diffraction data is presented. The program interacts with the launch mode kernel of the total powder pattern analysis software suite Topas® for doing the refinements. The program provides a graphical interface platform, upon which the huge Topas input command files necessary to perform sequential and parametric refinements can be easily prepared and executed. This program requires the user license dongle for Topas academic version 3 or higher.

Abstract: The first order phase transition of CsFeO2 was investigated using synchrotron powder diffraction data as a function of temperature. Two alternative approaches were used to describe the deviation of the framework crystal structure relative to the high-symmetry parent structure: symmetry (a.k.a. distortion) modes and polyhedral-tilt parameters. In both cases, the relevant parameters were refined as a function of temperature using the method of parametric Rietveld refinement. We demonstrate a semi-automated and generally applicable method for the determination of spontaneous lattice strain variations, order parameters and power-law exponents as derived from Landau theory.

Abstract: A simulated annealing like optimization method has been successfully applied for the determination of global minimum of equation of state (EoS) parameters. The approach is tested using the program TOPAS on a high precision variable pressure data set of quartz [R.J. Angel, D.R. Allan, R. Miletich, L.W. Finger: J. Appl. Cryst. Vol 30 (1997) p. 461] It is evidenced that least-squares refinement finds the global minimum for all EoS except for the Birch-Murnaghan EoS of fourth order for which it is shown that a clearly distinguished global minimum does not exist, at least for the used data set. The method presented herein serves as a fast test for the global minimum verification for the EoS parameters but should be applicable also to other optimisation problems.